Save battery consumption while browsing?

The color of the background will have little if any impact on battery life. There are many factors that impact your battery life. See the BATTERY LIFE FROM A CHARGE section of the following link for details, including tips on how to maximize your battery life.

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The link below should answer most, if not all, of your battery/charging questions. If you haven't already done so, I highly recommend you take the time to read it.

Apple Notebook Battery FAQ​

 

The background color and/or image is picked by the website's designer not Safari.

 

Can I get the same for iOS7?

 

It may not be an ideal solution, but press Command+Option+F5 to bring up the accessibility options and check the box next to "invert display colours". It will invert all the colours on your Mac, but at least the webpages will have black backgrounds. Just uncheck the box to disable.

You can also invert the display colours in iOS 7. Again, it's not ideal but it's the only viable option right now. Go to settings>general>accessibility and toggle the switch for "invert colours". It will invert everything on the phone, but at least it won't be as bright.